In the art of detecting components in fluids, cantilever based sensors with integrated piezoresistors are used as very sensitive mechanical stress sensors. As described in e.g. WO 0066266 and WO 9938007, micro cantilevers can be used for detection of molecular interaction. At least one surface of the cantilever is coated with a capture layer, which capture layer reacts with a target molecule of interest. If the cantilever is exposed to a sample in which the target molecule is present, the target molecule will react with the capture molecule on the cantilever surface and a surface stress change will be generated.
Due to the surface stress change of the cantilever, a mechanical compression, stretch or decompression is applied to the cantilever and thereby also to the piezoresistor, and thereby the resistivity of the piezoresistor changes its value. The mechanical compression or decompression may result in a deflection and/or a stretch and/or a contraction. By measuring the change in resistance, it can be determined whether the target molecule is present in the sample or not, and if so it may also be possibly to detect the concentration of the target molecule.
Cantilever-based sensors with integrated piezoresistive read-out are described by Thaysen, Ph.D. Thesis, “Cantilever for Bio-Chemical Sensing Integrated in a Microliquid Handling System”, September 2001, Microelektronik Centeret, Technical University of Denmark. Hereby the stress changes on the cantilever sensors can be measured directly by the piezoresistor. Moreover, integrated read-out greatly facilitates operation in solutions since the refractive indices of the fluids do not influence the detection as it will using optical read-out. Each sensor may have a built-in reference cantilever, which makes it possible to subtract background drift directly in the measurement. Furthermore, by functionalizing the reference cantilever with a “dummy” molecule, non-specific binding events occuring on both the measurement and reference cantilever will be cancelled out in the differential measurement.
The two cantilevers may be connected in a Whetstone bridge, and the stress change on the measurement cantilever is detected as the output voltage from the Whetstone Reference is also made to “Design issues in SOI-based high-sensitivity piezoresistive cantilever devices” by Kassegne et al. Proceedings of the SPIE Conference on Smart Structures and Materials, San Diego, Calif., Mar. 17-21 2002.
The objective of the present invention is to provide a sensor comprising one or more sensor units with a capture surface, which sensor can be used for detection of the presence of a target in a fluid, such as a chemical component in a liquid or a gas e.g. for detecting an explosive, a drug, a biocomponent or other components in a fluid with an improved signal or signal/noise ratio than according to the prior art cantilevered fluid sensors.
This and other objectives as it will be clear from the following description, has been solved by the invention as it is defined in the claims.